The present invention relates to differentials, and more particularly, to controllable, traction enhancing differentials.
Differentials are well known mechanisms and generally provide a means to transfer rotational torque, via an input shaft, i.e., a drive shaft, to a pair of output shafts, i.e., axle shafts. Conventional differential construction includes, typically, a fixed housing including a rotatable casing therein driven by the input shaft through a ring gear attached about the casing. The casing rotatably supports each output shaft which typically includes a side gear fixed thereto and positioned within the casing. The side gears intermesh with pinion gears which rotate about a pin fixed relative to the casing. Differentials are often utilized in conventional vehicle applications where the differential engages a pair of wheels which respectively mount to each output shaft to maintain traction with the road while the vehicle is turning. The differential essentially distributes torque, provided by the input shaft, to the output shafts. One type of differential, termed an xe2x80x9copenxe2x80x9d differential, includes a construction which distributes torque to the output shafts without implementing means to compensate for loss of traction. The open differential is unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel; for instance, when one wheel of the vehicle is located on a patch of ice and the other wheel is on dry pavement. The wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a xe2x80x9cspin outxe2x80x9d of that wheel. Since the maximum amount of torque which can be exerted on the wheel with traction is equal to torque on the wheel without traction, i.e., the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior methods of limiting slippage between the side gears and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, and a bias mechanism, usually a spring, to apply an initial preload between the side gears and the differential casing. By using a frictional clutch with an initial preload, for example a spring, a minimum amount of torque can always be applied to the wheel having traction, i.e. the wheel located on dry pavement. The initial torque generates gear separating forces which further act on the frictional clutch and develop additional torque. Examples of such limited slip differentials are disclosed in U.S. Pat. No. 4,612,825 (Engle), U.S. Pat. No. 5,226,861 (Engle) and U.S. Pat. No. 5,556,344 (Fox), which are assigned to the assignee of the present invention. The disclosures of these patents are each expressly incorporated herein by reference.
In a differential, the development of torque will create side gear separating forces which tend to move the side gears away from the pinion gears. In general, gear separating forces are forces induced on any set of meshing gears by the application of torque to the gears. Differentials were adapted to provide an initial preload to utilize side gear separating forces for further braking action between the side gears and the differential casing. In operation, when one wheel is in contact with a slippery surface, the initial preload creates contact and frictional engagement between the differential casing and a clutch mechanism. The clutch mechanism is disposed between the side gears and the differential casing to distribute engine torque to the wheel having traction. The torque transfer induces gear separating forces on the side gears tending to separate the side gears and further frictionally engage the clutch mechanism with the casing. The increased frictional engagement of the clutch allows more torque to be distributed between the side gears and the differential casing to effectively transfer torque to the wheel with traction. However, the clutches of such preloaded differentials are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Additionally, such clutch mechanisms usually employ spring mechanisms which add to the cost and difficulty of manufacture.
An additional problem associated with preloaded clutch mechanisms are that they lock the output shafts together in situations where differential rotation between axle shafts is necessary. For example, if the vehicle is making a turn when the wheels are sufficiently engaged on the road surface and a sufficient amount of torque is developed, the differential will tend to lock up the output shafts due to the action of the side gear separating forces. This may occur, for example, during turns on surfaces with a high coefficient of friction while under acceleration. In such a case, even though differential rotation is required, the two output shafts lock up causing one wheel to drag and slide along the road surface. This problem is evident in rear drive vehicles during turns under acceleration as the portion of the vehicle near the dragging wheel may tend to bounce up and down.
Another method of limiting slippage involves engaging a frictional clutch mechanism between the side gears and the differential casing based on the difference in rotational speeds between the two output shafts. Limited slip differentials employing this method are classified as speed-sensitive differentials. The frictional clutch may be actuated by various hydraulic pump mechanisms which may be external to the differential casing or may be constructed of elements disposed inside the differential casing. However, such mechanisms usually are complicated and also add to the cost and difficulty of manufacture. Further, speed sensitive differentials are xe2x80x9creactivexe2x80x9d, i.e., they react after a wheel has already lost traction.
Another known method of limiting slippage involves using a flyweight governor in combination with a clutch mechanism. The governor actuates the clutch mechanism when a predetermined differential rotation rate is detected. However, devices heretofore using such arrangements are configured such that the governor almost instantaneously applies extremely high clutch torque to the output shafts, which often leads to lock-up of the two output shafts. Distributing torque in such a manner applies very high stresses on the output shafts which may result in fracturing the output shafts.
In addition to actuating a clutch mechanism using mechanical or hydraulic arrangements, response and performance characteristics may be improved by controlling the actuation of a limited slip differential using electronic control methods. An example of such an electronically controlled differential is disclosed in U.S. Pat. No. 5,989,147 (Forrest), assigned to the assignee of the present invention, the disclosure of which is expressly incorporated herein by reference. Electronic control methods provide the advantage of accurate, reliable control within a narrow control band. Electronic control methods also allow operating parameters to be easily changed, for example by programming the electronic control systems to respond to a particular range of differentiation speeds or some other vehicle parameter such as throttle position.
The electronically controllable differential provides a clutch mechanism which transfers torque between a differential casing and a side gear in response to the application of an initiating force by an electronic actuator. The clutch mechanism, for example, may comprise a cone clutch element engageable with an insert disposed between the side gear and the rotatable casing. The clutch is engageable with the casing through camming portions provided between the side gear and clutch element. Alternatively, the camming portions may be substituted with a ball ramp assembly. The ball ramp assembly provides axial displacement of the clutch element when an initiating force is applied by the electronic actuator.
The electronic control system which actuates the differential typically includes the electronic actuator, sensors, which sense a predetermined rotational condition of the side gear, and an electromagnet which issues an electromagnetic field for applying the initiating force. The electromagnet is arranged to generate a generally toroidal magnetic flux path encircling the electromagnet to magnetically force the clutch into engagement with the casing.
In operation, specifically during non-slipping conditions, the electronically controllable differential operates as an open differential with the clutch disengaged from the housing. In slipping conditions, for example, a predetermined rotational condition of the differential components is sensed, the electronic control system actuates the electromagnet to issue a magnetic field which applies an initiating force to the clutch. The initiating force produces an initial axial movement of the clutch such that the clutch, through frictional engagement, momentarily slows down with respect to the side gear. The momentary slowdown further effectuates torque transfer through axial displacement of the side gear to provide a predetermined amount of torque from the rotatable casing to the side gear. Both ball and ramp arrangements and interacting cam portion type differentials are equally adaptable to electronically controllable differentials.
Although utilizing an electromagnet to induce frictional force between the clutch and the differential casing provides an increased level of controllability over a limited slip differential, and ultimately, a more precise accountability of torque to each output shaft, there are certain disadvantages inherent in such a design. One such disadvantage includes the increased mechanical hysteresis associated with frictional component clutches. Increased hysteresis is a consequence of the significant force requirement associated with separating and engaging the clutch element. These force requirements may be unpredictable due to several factors which include: heating of engaged surfaces, part wear and clutch seizure. Thus, a limited slip differential having improved mechanical hysteresis would be highly desirable. Further, another disadvantage of frictional clutch type differentials is the diminished controllability depending on wear of the torque transferring components. A further disadvantage includes the complexity required to machine the frustoconical, frictional engaging components which adds significant manufacturing cost to the differential.
Mechanisms used heretofore, in association with exercise equipment and engine mount applications, include magnetorheological (MR) fluid mechanisms replacing traditionally used dampers, shock absorbers and resistance elements, i.e., springs. For example, the MR fluid damper is constructed of a housing reciprocally supporting a plunger submersed in the MR fluid. In operation, a magnetic field is introduced to the MR fluid within the housing to transform or solidify the fluid which effectuates a resistance on the plunger and a desirable damping effect is experienced.
The MR fluid includes magnetic particles dispersed or suspended in a carrier fluid. The carrier fluid typically has a viscosity similar to that of engine oil. In the presence of a magnetic field, however, the magnetic particles become polarized and are thereby organized into chains of particles within the carrier fluid. The chains of particles effectuate an increase in the viscosity or flow resistance of the fluid resulting in the development of a substantially solid mass, the viscosity thereof similar to that of a Bingham solid. Bingham solids have a zero rate of flow in the presence of a shear producing a stress in the material less than the material""s yield strength and a linear rate of flow when the shear produces a stress above the material""s yield strength. The Bingham solid returns to liquid when the magnetic field is removed due to the particles returning to an unorganized and suspended state within the carrier fluid.
It is desirable to enhance controllability of the torque transfer between axle shafts and the housing of an electrically controllable differential. Furthermore, it is desirable to decrease, in a limited slip differential, the number of friction wearing components otherwise requiring costly maintenance or replacement.
The present invention provides a limited slip differential including a rotating casing, first and second side gears disposed within the casing, at least one pinion gear disposed within the casing and engaged with the first and second side gears, the pinion gear rotatably attached to the casing, a quantity of MR fluid provided between the first side gear and the casing, and a magnet from which a magnetic field is issued, the MR fluid being selectively exposed to the magnetic field. The first side gear and the casing are rotatably coupled through the MR fluid when the MR fluid is exposed to the magnetic field, whereby relative rotation between the first and second side gears is controlled.
The present invention also provides a limited slip differential including a rotating casing, at least one pinion gear rotatably attached to the casing, and first and second axles extending into the casing, the axles being engaged with, and rotatable relative to, each other and to the casing. At least one axle is also rotatably coupled to rotating casing, whereby that axle is driven by the casing. Means are also provided for selectively increasing the viscosity of a magnetic fluid in operative engagement with an axle and the casing, and rotatably coupling both axles to the casing in response to the viscosity increase, whereby both axles are driven by the casing.
The present invention further provides method for operating a limited slip differential, including rotating a first axle relative to a rotating casing and a second axle, applying a magnetic field to an MR fluid, whereby the viscosity of the MR fluid is increased, and rotatably coupling at least one of the first and second axles to the rotating casing through the increased viscosity MR fluid, whereby the relative rotation is slowed.
The present invention also provides a limited slip differential including a rotatable casing, a pair of axle shafts including respective ends, a pair of side gears rotatably fixed to the ends of the axle shafts, at least one pinion gear attached to the casing and meshingly engaged with the pair of side gears, a brake assembly defining a brake chamber and including first and second brake elements disposed within the brake chamber, the first brake element superposed with the second brake element, the first brake element rotatably fixed relative to the casing, a quantity of magnetorheological fluid disposed within the brake chamber, the first and second brake elements in contact with the magnetorheological fluid, and a selectively energized source of magnetic flux, the magnetorheological fluid being exposed to the flux when the source is energized. When the magnetorheological fluid is exposed to the magnetic flux, the magnetorheological fluid is at least partially solidified and the first and second brake elements are coupled to each other through the magnetorheological fluid.
Notably, the magnetic flux may be variable. Further, in some embodiments, the differential may include a clutch which is rotatably fixed relative to one of the axle shafts and releaseably operatively engaged with the casing, and means for engaging the clutch in response to relative rotation between portions of the brake assembly and the other axle shaft.
The differential of the present invention decreases the mechanical hysteresis associated with utilizing frictional clutch assemblies to transfer torque between a casing and an axle shaft, as in prior limited slip differentials. The present invention, by decreasing mechanical hysteresis, decreases the time of control response of the differential.
Another advantage of the present invention is that wear and fatigue, inherent in frictional torque transferring components of prior limited slip differentials, is significantly decreased due to the decrease in the number of frictional torque transferring members required.
A further advantage of the present invention is a decrease in number of frictionally engaging clutch components vis-a-vis prior limited slip differentials. The components are complex to manufacture, and the reduction in their number represents significant manufacturing cost savings.